Method for the production of liquefied natural gas

ABSTRACT

A method for the production of liquefied natural gas is provided. The method may include providing a high pressure natural gas stream, splitting the high pressure natural gas stream into a first portion and a second portion, and liquefying the first portion of the high pressure natural gas stream to produce an LNG stream. The refrigeration needed for cooling and liquefaction of the natural gas can be provided by a closed nitrogen refrigeration cycle and letdown of the second portion of the high pressure natural gas stream.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application Ser. No. 62/201,947, filed on Aug. 6, 2015, U.S. Provisional Patent Application No. 62/305,381, filed on Mar. 8, 2016, and U.S. Provisional Application Ser. No. 62/370,953 filed on Aug. 4, 2016, all of which are hereby incorporated by reference in their entireties.

TECHNICAL FIELD OF THE INVENTION

The present invention generally relates to a method for efficiently producing liquefied natural gas (LNG).

BACKGROUND OF THE INVENTION

Many locations utilize a high pressure (transmission) network and a lower pressure (distribution) network to supply natural gas through a local area. The transmission network typically acts as a freeway to economically send the gas over long distances to the general area, while the distribution network acts as the roads to send the gas to the individual users within a local area. Pressures of these networks vary by location, but typical values are between 30-80 bara for transmission and 3-20 bara for distribution. Some applications (e.g., cogeneration, boilers, etc. . . . ) have high flowrates of natural gas and other utilities such as nitrogen which are letdown to the consumer or to the lower pressure network at relatively constant flow, pressure and temperature conditions. This pressure letdown energy is often not utilized.

Traditionally natural gas is compressed and sent down pipelines under high pressure to transport the gas to market. High pressures are used in order to reduce the volumetric flow of the gas thereby reducing pipe diameters (capex) and/or compression energy related to pressure losses (opex). Pipeline operators also utilize the high pressure as a buffer to accommodate transient demands. When the gas has arrived at its use point, the pressure of the natural gas is reduced in one or more control valves to its final pressure for consumption. The available energy from the reduction in pressure of the natural gas is wasted in the control valves as well as any chilling effect (also known as the Joule Thomson effect) caused by the flow of natural gas through these devices. Such systems often require heaters and condensate systems due to the colder conditions of the downstream gas.

In the past, advantage has been taken of this wasted energy by facilities utilizing the energy and refrigeration effect of expanding the natural gas. One such facility was designed and constructed by Airco Industrial Gases' Cryoplants Division in the early 1970's in Reading, Pa. for UGI Corporation. It employed a natural gas pressure reduction station (“Letdown Station”) to make liquefied natural gas (“LNG”) or liquid nitrogen (“LIN”). A majority of the natural gas entering the plant under high pressure from the transportation pipeline was cooled and sent to an expansion turbine where energy and refrigeration were generated. The remainder of the stream was subsequently cooled with the refrigeration and a portion liquefied. The liquefied portion was then passed to a storage tank as LNG product. The natural gas that was not liquefied was warmed, collected and sent to the low pressure main at a lower pressure than the high-pressure main.

U.S. Pat. No. 6,196,021 describes a system that uses natural gas expansion to provide refrigeration to liquefy a natural gas stream which is then vaporized by heat exchange with a nitrogen stream to cool the nitrogen stream. This refrigeration supplements refrigeration provided by nitrogen pressure letdown and a nitrogen cycle to provide liquid nitrogen. Similarly, U.S. Pat. No. 6,131,407 describes a system that produces LIN to be sent directly to an air separation unit (“ASU”) to assist refrigeration of the ASU. U.S. Patent Application Publication No. 2014/0352353 describes a similar system to the system of disclosed by U.S. Pat. No. 6,131,407, but adds that the LIN produced can be sent to a tank instead of being used to liquid assist the ASU. In each of these systems, LNG is revaporized to provide for nitrogen cooling. However, it is not desirable to liquefy and then revaporize the natural gas, as this is thermally inefficient. U.S. Pat. No. 6,694,774 describes a system that uses natural gas letdown to provide refrigeration to produce a liquefied natural gas stream, where the refrigeration is supplemented by a closed loop mixed refrigerant cycle.

FIG. 1 provides a process flow diagram for a typical small LNG scheme that utilizes a nitrogen cycle 50, which includes nitrogen compressor 10, coolers 11, 21, 26, and first and second turbine boosters 20, 25, in a closed loop. For purposes discussed herein, a turbine booster is a combination of a turbine and a booster, in which the booster is powered, at least partially, by the turbine, which is typically accomplished via a common shaft. Natural gas 2 is first purified of components that would damage equipment or freeze during liquefaction in purification unit 30. Purified natural gas 4 is then cooled in heat exchanger 40, where it is condensed into LNG 6 using refrigeration provided by the nitrogen refrigeration cycle 50. Typically, heavy hydrocarbons (pentane and heavier) are removed from the natural gas either before or from an intermediate location of the exchanger 40 by adsorption, distillation or gas-liquid separator in order to prevent these components from freezing in the exchanger 40. In the example of FIG. 1 the natural gas 4 is withdrawn from an intermediate section of the heat exchanger 40 in order to remove the heavy hydrocarbons 8 using gas liquid separator 5. In the typical setup shown of FIG. 1, the power required to produce 342 mtd of LNG is approximately 7155 kW, meaning the specific power of this setup is approximately 502 kWh/mt.

Therefore, it would be advantageous to provide a method and apparatus that operated in a more efficient manner yielding a lower cost of LNG.

SUMMARY OF THE INVENTION

The present invention is directed to a method and apparatus that satisfies at least one of these needs. In certain embodiments, the invention can provide a lower cost, more efficient and flexible method to produce LNG. For example, in certain embodiments, the invention can also include coproduction of liquid nitrogen (“LIN”). In additional embodiments, the invention may include varying the production rates of either or both the LIN and LNG, based on power costs, product demand, and/or supply levels.

Nitrogen is transported through high pressure pipelines because of the lower transport cost of reduced volumetric flows associated with high pressure gas. Typically such pipelines operate in the range of 30 to 50 bara. Customers using nitrogen from a pipeline often do not need the nitrogen at these pressures. For example, nitrogen is typically used as an inert utility fluid at pressures in range of 3 to 8 bara. As such, in these locations, potential refrigeration capacity is wasted. Additionally, there are instances in which producers of the nitrogen gas feeding the pipeline do not operate at 100% of equipment design capacity, and therefore, large nitrogen compressors are either not operating or not operating at optimum capacity. This can occur if the demand for nitrogen is lower than originally anticipated, for example. Another reason this occurs is because the nitrogen producing equipment is sized to meet peak customer demand under peak operating scenarios, ambient conditions, catalyst life, and the like. As such, the nitrogen producing equipment may be designed to be underutilized during many operating scenarios when other systems are not able to accommodate increased loads.

In certain embodiments of the invention, a process can provide for LNG and/or LIN production with at least reduced energy input by using the refrigeration capabilities of letdown of natural gas and let down of nitrogen or a gas rich in nitrogen. An example of a gas rich in nitrogen is a lean synthetic air stream with less than 12% O₂ (e.g., due to the limit of combustion for a mixture with methane). In embodiments, the letdown process occurs at a location that is proximate to an existing facility or location where the letdown of both natural gas and nitrogen occurs to serve the needs of the facility, such that LNG and/or LIN can be produced with reduced operating costs and/or capital costs as compared to a situation without the benefit of the letdown of a gas stream (e.g., a nitrogen stream, a stream of gas rich in nitrogen, or a natural gas or other high pressure gas stream at a production site).

In one embodiment, the invention can include a method for the production of liquefied natural gas (“LNG”). In one embodiment, the method can include the steps of: a) providing a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle is configured to provide refrigeration within a heat exchanger; b) purifying a first natural gas stream in a first purification unit to remove a first set of impurities to produce a purified first natural gas stream; c) cooling and liquefying the first natural gas stream in the heat exchanger using the refrigeration from the nitrogen refrigeration cycle to produce an LNG stream, wherein the first natural gas stream has an LNG refrigeration requirement, wherein the LNG stream is liquefied at a first pressure P_(H); d) purifying a second natural gas stream in a second purification unit to remove a second set of impurities to produce a purified second natural gas stream; e) partially cooling the second natural gas stream in the heat exchanger; f) withdrawing the partially cooled second natural gas stream from an intermediate section of the heat exchanger; g) expanding the partially cooled second natural gas stream to a medium pressure P_(M) in a natural gas expansion turbine to form a cold natural gas stream, wherein the medium pressure P_(M) is at a pressure lower than the first pressure P_(H); and h) warming the cold natural gas stream in the heat exchanger by heat exchange against the first natural gas stream to produce a warm natural gas stream at the warm end of the heat exchanger, wherein the natural gas expansion turbine drives a first booster, wherein the LNG refrigeration requirement is supplied by a combination of refrigeration from the nitrogen refrigeration cycle and step h).

In optional embodiments of the method for the production of LNG:

-   -   the first booster is configured to compress the second natural         gas stream or a stream derived from the second natural gas         stream;     -   the first booster is configured to compress a stream selected         from the group consisting of the first natural gas stream, the         first purified natural gas stream, the second natural gas         stream, the purified second natural gas stream, the partially         cooled natural gas stream, the warm natural gas stream, and a         nitrogen fluid within the nitrogen refrigeration cycle;     -   the first set of impurities has a freezing point at or above the         liquefaction temperature of methane at the first pressure P_(H);     -   the second set of impurities comprises water;     -   the nitrogen refrigeration cycle comprises a recycle compressor,         a turbine, a booster and a plurality of coolers, wherein the         turbine and booster are configured such that the turbine is         configured to power the booster;     -   the first natural gas stream and the second natural gas stream         come from the same natural gas source;     -   the natural gas source is a natural gas pipeline having a         pressure between 15 and 100 bara;     -   the first natural gas stream comes from a first natural gas         source, and the second natural gas stream comes from a second         natural gas source, wherein the first and second natural gas         sources are different sources;     -   the first natural gas source comprises a natural gas pipeline;     -   the natural gas pipeline has a pressure between 15 and 100 bara;     -   the first purification unit and the second purification unit are         the same unit; and/or     -   the first purification unit and the second purification unit are         separate units, wherein the first purification unit is         configured to remove at least water and carbon dioxide, and         wherein the second purification unit is configured to remove at         least water.

In another aspect of the invention, a method for the production of liquefied natural gas (“LNG”) is provided. In this embodiment, the method comprising the steps of: a) providing a nitrogen refrigeration cycle; b) cooling and liquefying a first natural gas stream in a heat exchanger by heat exchange with nitrogen from the nitrogen refrigeration cycle to produce an LNG stream, wherein the LNG stream is liquefied at a first pressure; c) expanding a second natural gas stream to a second pressure to produce an expanded natural gas stream; and d) warming the expanded natural gas stream in the heat exchanger to produce a warmed natural gas stream, wherein step d) provides a portion of the refrigeration used to cool and liquefy the first natural gas stream.

In optional embodiments of the method for the production of LNG:

-   -   the first natural gas stream comes from a first natural gas         source, and the second natural gas stream comes from a second         natural gas source, wherein the first and second natural gas         sources are different sources; and/or     -   the first natural gas liquefied in step b) is derived from the         expanded natural gas stream, wherein the first pressure and the         second pressure are about the same.

In another aspect of the invention, a method for the production of liquefied natural gas (“LNG”) is provided. In this embodiment, the method comprising the steps of a) providing a high pressure natural gas stream; b) splitting the high pressure natural gas stream into a first portion and a second portion; c) cooling and liquefying the first portion of the high pressure natural gas stream to produce an LNG stream; d) providing a first portion of refrigeration via a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle comprises a recycle compressor, a turbine, a booster and a plurality of coolers, wherein the turbine and booster are configured such that the turbine is configured to power the booster; e) providing a second portion of refrigeration by expanding the second portion of the high pressure natural gas; and f) using the first portion of refrigeration and the second portion of refrigeration to achieve the cooling and liquefaction of the first portion of the high pressure natural gas stream in step c).

In another aspect of the invention, a method for the production of liquefied natural gas (“LNG”) and liquid nitrogen (“LIN”) is provided. In this embodiment, the method can include the steps of: a) providing a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle is configured to provide refrigeration within a heat exchanger, wherein a portion of the nitrogen within the nitrogen refrigeration cycle is withdrawn and liquefied yielding a liquid nitrogen product, wherein at least an equal portion of gaseous nitrogen is introduced to the nitrogen refrigeration cycle as is withdrawn; b) purifying a first natural gas stream in a first purification unit to remove a first set of impurities to produce a purified first natural gas stream; c) cooling and liquefying the first natural gas stream in the heat exchanger using the refrigeration from the nitrogen refrigeration cycle to produce an LNG stream, wherein the first natural gas stream has an LNG refrigeration requirement, wherein the LNG stream is liquefied at a first pressure P_(H); d) purifying a second natural gas stream in a second purification unit to remove a second set of impurities to produce a purified second natural gas stream; e) partially cooling the second natural gas stream in the heat exchanger; f) withdrawing the partially cooled second natural gas stream from an intermediate section of the heat exchanger; g) expanding the partially cooled second natural gas stream to a medium pressure P_(M) in a natural gas expansion turbine to form a cold natural gas stream, wherein the medium pressure P_(M) is at a pressure lower than the first pressure P_(H); and h) warming the cold natural gas stream in the heat exchanger by heat exchange against the first natural gas stream to produce a warm natural gas stream at the warm end of the heat exchanger, wherein the natural gas expansion turbine drives a first booster, wherein the LNG refrigeration requirement is supplied by a combination of refrigeration from the nitrogen refrigeration cycle and step h).

In optional embodiments of the method for the production of LNG and LIN:

-   -   the first booster is configured to compress the second natural         gas stream or a stream derived from the second natural gas         stream;     -   the first booster is configured to compress a stream selected         from the group consisting of the first natural gas stream, the         purified first natural gas stream; the second natural gas         stream, the purified second natural gas stream, the partially         cooled natural gas stream, the warm natural gas stream, and a         nitrogen fluid within the nitrogen refrigeration cycle;     -   the liquid nitrogen product has a LIN refrigeration requirement,         wherein the LIN refrigeration requirement is supplied by a         combination of refrigeration from the nitrogen refrigeration         cycle and step h);     -   the first set of impurities has a freezing point at or above the         liquefaction temperature of methane at the first pressure P_(H);     -   the second set of impurities comprises water;     -   the nitrogen refrigeration cycle comprises a recycle compressor,         a turbine, a booster and a plurality of coolers, wherein the         turbine and booster are configured such that the turbine is         configured to power the booster;     -   the nitrogen refrigeration cycle further comprises a nitrogen         feed compressor;     -   the first natural gas stream and the second natural gas stream         come from the same natural gas source;     -   the natural gas source is a natural gas pipeline having a         pressure between 15 and 100 bara;     -   the first natural gas stream comes from a first natural gas         source, and the second natural gas stream comes from a second         natural gas source, wherein the first and second natural gas         sources are different sources;     -   the first natural gas source comprises a natural gas pipeline;     -   the natural gas pipeline has a pressure between 15 and 100 bara;     -   the first purification unit and the second purification unit are         the same unit;     -   the first purification unit and the second purification unit are         separate units, wherein the first purification unit is         configured to remove at least water and carbon dioxide, and         wherein the second purification unit is configured to remove at         least water; and/or     -   the nitrogen liquefier further comprises a subcooler.

In another aspect of the invention, a method for the production of liquefied natural gas (“LNG”) and liquid nitrogen (“LIN”) is provided. In this embodiment, the method can include the steps of: a) providing a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle is configured to provide refrigeration within a heat exchanger, wherein a portion of the nitrogen within the nitrogen refrigeration cycle is withdrawn and liquefied yielding a liquid nitrogen product, wherein at least an equal portion of gaseous nitrogen is introduced to the nitrogen refrigeration cycle as is withdrawn; b) cooling and liquefying a first natural gas stream in a heat exchanger by heat exchange with nitrogen from the nitrogen refrigeration cycle to produce an LNG stream, wherein the LNG stream is liquefied at a first pressure; c) expanding a second natural gas stream to a second pressure to produce an expanded natural gas stream; and d) warming the expanded natural gas stream in the heat exchanger to produce a warmed natural gas stream, wherein step d) provides a portion of the refrigeration used to cool and liquefy the first natural gas stream.

In optional embodiments of the method for the production of LNG and LIN:

-   -   the first natural gas stream comes from a first natural gas         source, and the second natural gas stream comes from a second         natural gas source, wherein the first and second natural gas         sources are different sources;     -   the liquid nitrogen product has a LIN refrigeration requirement,         wherein the LIN refrigeration requirement is supplied by a         combination of refrigeration from the nitrogen refrigeration         cycle and step d); and/or     -   the first natural gas liquefied in step b) is derived from the         expanded natural gas stream, wherein the first pressure and the         second pressure are about the same.

In another aspect of the invention, a method for the production of liquefied natural gas (“LNG”) and liquid nitrogen (“LIN”) is provided. In this embodiment, the method can include the steps of: a) providing a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle comprises a recycle compressor, a turbine, a booster and a plurality of coolers, wherein the turbine and booster are configured such that the turbine is configured to power the booster, wherein a portion of the nitrogen within the nitrogen refrigeration cycle is withdrawn and liquefied yielding a liquid nitrogen product, wherein at least an equal portion of gaseous nitrogen is introduced to the nitrogen refrigeration cycle as is withdrawn; b) providing a high pressure natural gas stream; c) splitting the high pressure natural gas stream into a first portion and a second portion; d) cooling and liquefying the first portion of the high pressure natural gas stream to produce an LNG stream; e) providing a first portion of refrigeration via the nitrogen refrigeration cycle; f) providing a second portion of refrigeration by expanding the second portion of the high pressure natural gas; and g) using the first portion of refrigeration and the second portion of refrigeration to achieve the cooling and liquefaction of the first portion of the high pressure natural gas stream in step d).

In another aspect of the invention, a method for the integration of a nitrogen liquefier and natural gas liquefier for the production of liquefied natural gas (“LNG”) and liquid nitrogen (“LIN”) is provided. In this embodiment, the method can include the steps of: a) providing a nitrogen liquefier having a first nitrogen refrigeration cycle, wherein the nitrogen liquefier comprises a turbine, a booster and a plurality of coolers, wherein the first nitrogen refrigeration cycle is configured to provide refrigeration within a first heat exchanger; b) providing a second nitrogen refrigeration cycle, wherein the second nitrogen refrigeration cycle comprises a second turbine, a second booster and a plurality of second coolers, wherein the second nitrogen refrigeration cycle is configured to provide refrigeration within a second heat exchanger; c) purifying a first natural gas stream in a first purification unit to remove a first set of impurities to produce a purified first natural gas stream; d) cooling and liquefying the first natural gas stream in the second heat exchanger using the refrigeration from the nitrogen refrigeration cycle to produce an LNG stream, wherein the first natural gas stream has an LNG refrigeration requirement, wherein the LNG stream is liquefied at a first pressure P_(H); e) purifying a second natural gas stream in a second purification unit to remove a second set of impurities to produce a purified second natural gas stream; f) partially cooling the second natural gas stream in the second heat exchanger; g) withdrawing the partially cooled natural gas stream from an intermediate section of the second heat exchanger; h) expanding the partially cooled natural gas stream to a medium pressure P_(M) in a natural gas expansion turbine to form a cold natural gas stream, wherein the medium pressure P_(M) is at a pressure lower than the first pressure P_(H); and i) warming the cold natural gas stream in the second heat exchanger by heat exchange against the first natural gas stream to produce a warm natural gas stream at the warm end of the second heat exchanger, wherein the natural gas expansion turbine drives a first booster, wherein the LNG refrigeration requirement is supplied by a combination of refrigeration from the second nitrogen refrigeration cycle and step i), wherein a portion of the liquid nitrogen within the first nitrogen refrigeration cycle is withdrawn as product liquid nitrogen, wherein at least an equal portion of gaseous nitrogen is introduced to the first nitrogen refrigeration cycle as is withdrawn as product liquid nitrogen, and wherein the first nitrogen refrigeration cycle and the second nitrogen refrigeration cycle share a common nitrogen recycle compressor.

In optional embodiments of the method integration of a nitrogen liquefier and natural gas liquefier:

-   -   the first booster is configured to compress the second natural         gas stream or a stream derived from the second natural gas         stream;     -   the first booster is configured to compress a stream selected         from the group consisting of the first natural gas stream, the         purified first natural gas stream, the second natural gas         stream, the purified second natural gas stream, the partially         cooled natural gas stream, the warm natural gas stream, and a         nitrogen fluid within the nitrogen refrigeration cycle; the         first set of impurities has a freezing point at or above the         liquefaction temperature of methane at the first pressure P_(H);     -   the second set of impurities comprises water;     -   the first nitrogen refrigeration cycle further comprises a         nitrogen feed compressor;     -   the first nitrogen refrigeration cycle is a closed refrigeration         cycle;     -   the first natural gas stream and the second natural gas stream         come from the same natural gas source;     -   the natural gas source is a natural gas pipeline having a         pressure between 15 and 100 bara;     -   the first natural gas stream comes from a first natural gas         source, and the second natural gas stream comes from a second         natural gas source, wherein the first and second natural gas         sources are different sources;     -   the first natural gas source comprises a natural gas pipeline;     -   the natural gas pipeline has a pressure between 15 and 100 bara;     -   the first purification unit and the second purification unit are         the same unit;     -   the first purification unit and the second purification unit are         separate units, wherein the first purification unit is         configured to remove at least water and carbon dioxide, and         wherein the second purification unit is configured to remove at         least water; and/or     -   the nitrogen liquefier further comprises a subcooler.

In another aspect of the invention, a method for the integration of a first liquefier and a second liquefier for the production of first liquefied gas and a second liquefied gas is provided. In this embodiment, the method can include the steps of: a) providing a first liquefier having a first refrigeration cycle, wherein the first liquefier comprises a recycle compressor, a first heat exchanger, and a turbine booster; b) providing a second refrigeration cycle, wherein the second refrigeration cycle is configured to provide refrigeration within a second heat exchanger, c) cooling and liquefying a first gas stream in the second heat exchanger by heat exchange with the second refrigeration cycle to produce a liquefied first gas stream, wherein the liquefied first gas stream is at a first pressure; d) expanding a second gas stream to a second pressure to produce an expanded second gas stream; and e) warming the expanded second gas stream in the second heat exchanger to produce a warmed gas stream, wherein a portion of a first refrigeration gas within the first refrigeration cycle is withdrawn and liquefied yielding a liquid first refrigeration gas product, wherein at least an equal portion of gaseous first refrigeration gas is introduced to the first refrigeration cycle as is withdrawn as liquid first refrigeration gas product, wherein step e), in addition to the refrigeration provided by the second refrigeration cycle, provides the refrigeration used to cool and liquefy the first gas stream, and wherein the first refrigeration cycle and the second refrigeration cycle share a common recycle compressor.

In optional embodiments of the method for the integration of a first liquefier and a second liquefier:

-   -   the first refrigeration cycle is selected from the group         consisting of a nitrogen refrigeration cycle and a hydrogen         refrigeration cycle;     -   the first gas stream liquefied in step c) is derived from the         expanded second gas stream, wherein the first pressure and the         second pressure are about the same;     -   the second refrigeration cycle is selected from the group         consisting of a nitrogen refrigeration cycle and a hydrogen         refrigeration cycle;     -   the first gas stream cooled and liquefied in step c) comprises         natural gas;     -   the second gas stream expanded in step d) comprises natural gas;         and/or     -   the liquid first refrigeration gas product is liquid nitrogen.

In another aspect of the invention, a method for the integration of a first liquefier and a second liquefier for the production of a first liquefied gas and a second liquefied gas is provided. In this embodiment, the method can include the steps of: a) providing a first liquefier having a first refrigeration cycle using a first refrigerant, wherein the first refrigeration cycle is configured to provide refrigeration within a first heat exchanger; b) providing a second liquefier having a second refrigeration cycle using a second refrigerant, wherein the second refrigeration cycle is configured to provide refrigeration within a second heat exchanger; c) cooling a first gas stream in the first heat exchanger by heat exchange with the first refrigeration cycle to produce a cooled first gas stream; d) cooling a second gas stream in the second heat exchanger by heat exchange with the second refrigeration cycle to produce a cooled second gas stream; e) expanding a third gas stream to produce an expanded third gas stream; and f) warming the expanded third gas stream in a heat exchanger selected from the group consisting of the first heat exchanger, the second heat exchanger, and combinations thereof, to produce a warmed gas stream, wherein step f), in addition to the refrigeration provided by the second refrigeration cycle, provides the refrigeration used to cool the second gas stream, wherein step f), in addition to the refrigeration provided by the first refrigeration cycle, provides the refrigeration used to cool the first gas stream, and wherein the first refrigeration cycle and the second refrigeration cycle share a common recycle compressor.

In optional embodiments of the method for the integration of a first liquefier and a second liquefier:

-   -   the first and second refrigeration cycles are nitrogen         refrigeration cycles;     -   the first refrigerant and the second refrigerant have the same         composition;     -   the first gas stream is selected from the group consisting of         natural gas, ethane, ethylene, acetylene, other C3-C6 alkanes,         alkenes, and alkynes, and nitrogen, and wherein the first gas         stream is liquefied during cooling step c);     -   the first gas stream is selected from the group consisting of         hydrogen and helium, wherein the first gas stream is not         liquefied during cooling step c);     -   the third gas stream expanded in step e) comprises natural gas;     -   a portion of the first refrigerant within the first         refrigeration cycle is withdrawn and liquefied yielding a liquid         first refrigerant product, wherein at least an equal portion of         gaseous first refrigerant is introduced to the first         refrigeration cycle as is withdrawn as liquid first refrigerant;         and/or     -   a portion of the second refrigerant within the second         refrigeration cycle is withdrawn and liquefied yielding a liquid         second refrigerant, wherein at least an equal portion of the         second refrigerant is introduced to the second refrigeration         cycle as is withdrawn as liquid second refrigerant.

In another aspect of the invention, a method for the integration of a nitrogen liquefier and letdown of natural gas for the production liquid nitrogen (“LIN”) is provided. In this embodiment, the method can include the steps of: a) providing a nitrogen liquefier having a nitrogen refrigeration cycle, wherein the nitrogen liquefier comprises a nitrogen recycle compressor, a heat exchanger, and a first turbine booster; b) introducing a nitrogen gas stream to the nitrogen liquefier under conditions effective for liquefying the nitrogen to produce a liquid nitrogen product; c) withdrawing a natural gas stream from a source operating at a first pressure P_(H); d) purifying the natural gas stream in a purification unit to produce a purified natural gas; e) partially cooling the purified natural gas in the heat exchanger; f) withdrawing the partially cooled natural gas from an intermediate section of the heat exchanger; g) expanding the partially cooled natural gas to a medium pressure P_(M) in a natural gas expansion turbine to form a cold natural gas stream, wherein the medium pressure P_(M) is at a pressure lower than the first pressure P_(H); and h) warming the cold natural gas stream in the heat exchanger by heat exchange against nitrogen from the nitrogen refrigeration cycle to produce a warm natural gas stream from a warm end of the heat exchanger, wherein step h) provides additional refrigeration to the nitrogen liquefier such that additional liquid nitrogen can be produced as compared to a method having an absence of steps c)-i), wherein the natural gas expansion turbine drives a first gas booster.

In optional embodiments of the method for the integration of a nitrogen liquefier and letdown of a natural gas stream:

-   -   the first gas booster is configured to compress the natural gas         stream or a stream derived therefrom;     -   the first gas booster is configured to compress a stream         selected from the group consisting of the natural gas stream,         the purified natural gas stream, the partially cooled natural         gas stream, the warm natural gas stream, and a nitrogen fluid         within the nitrogen refrigeration cycle;     -   the purification unit is configured to remove at least water         from the natural gas stream;     -   the nitrogen refrigeration cycle further comprises a nitrogen         feed compressor;     -   the nitrogen liquefier further comprises a subcooler;     -   the source of the natural gas comprises a natural gas pipeline;         and/or     -   the natural gas pipeline has a pressure between 15 and 100 bara.

In another aspect of the invention, a method for integration of a nitrogen liquefier and letdown of natural gas for the production of liquid nitrogen (“LIN”) is provided. In this embodiment, the method can include the steps of: a) providing a nitrogen liquefier having a nitrogen refrigeration cycle, wherein the nitrogen liquefier comprises a nitrogen recycle compressor, a heat exchanger, and at least one turbine booster; b) introducing a nitrogen gas stream to the nitrogen liquefier under conditions effective for liquefying the nitrogen to produce a liquid nitrogen product; c) recovering a natural gas stream from a high pressure source, wherein the natural gas stream is at a first pressure; d) expanding the natural gas stream to a second pressure to produce an expanded natural gas stream, wherein the second pressure is a pressure that is lower than the first pressure; and e) warming the expanded natural gas stream in the heat exchanger to produce a warmed natural gas stream, wherein step e) provides additional refrigeration to the nitrogen liquefier such that additional liquid nitrogen can be produced as compared to a method having an absence of steps d) through e).

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.

FIG. 1 provides an embodiment of the prior art.

FIG. 2 provides an embodiment of the present invention.

FIG. 3 provides an embodiment of the present invention with both LIN and LNG production.

FIG. 4 provides another embodiment of the present invention with both LIN and LNG production.

FIG. 5 provides an embodiment of the present invention with LIN and medium pressure natural gas production.

DETAILED DESCRIPTION

While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.

In one embodiment, the method can include integrating a natural gas letdown system with a nitrogen refrigeration cycle. In one embodiment, the nitrogen refrigeration cycle is a closed loop refrigeration cycle. In this embodiment, the natural gas letdown essentially provides “free” refrigeration energy since the natural gas would have been alternatively letdown across a valve (i.e., the resulting drop in temperature of the natural gas would have been absorbed by the surroundings and would not have been recovered in any meaningful way). With the addition of a natural gas turbine booster, LNG can be co-produced with a significant power savings, while also potentially reducing the size of the nitrogen refrigeration cycle. In another embodiment, a purification unit, storage, loading and utility systems may also be included.

Referring to FIG. 2, a process flow diagram of an embodiment of the current invention is shown. In FIG. 2, high pressure natural gas 2 is preferably split into two portions, with one portion being liquefied and the other portion providing a portion of the refrigeration used to cool and liquefy the natural gas. First portion of the natural gas stream 102 is purified in first purification unit 130, wherein acid gases, water and mercury are preferably removed. Preferably, any impurity within the natural gas that would solidify prior to the natural gas liquefying or damage the downstream equipments is removed in first purification unit 130. The resulting purified first portion of the natural gas stream 104 is then withdrawn from first purification unit 130 and introduced to heat exchanger 40 for liquefaction therein. In embodiments in which the natural gas feed contains heavy hydrocarbons, it is preferable to withdraw purified first portion of the natural gas 104 from an intermediate section of heat exchanger 40 and separate the heavy hydrocarbons 8 using gas liquid separator 5. Alternatively, the gas-liquid separator may be replaced by a distillation column or other separation devices known in the art. Instead of collecting heavy hydrocarbons 8 separately as shown in FIG. 1, heavy hydrocarbons 8 may be expanded and then warmed in heat exchanger 40. The resulting warmed stream can be combined with other natural gas streams (e.g., cold natural gas stream 144 and first portion of the LNG 146) within heat exchanger 40. This advantageously captures some of the cold energy from heavy hydrocarbons 8, and if warm natural gas stream 108 is subsequently used for fuel, it also provides additional energy for that purpose.

Vaporized natural gas from gas liquid separator 5 is reintroduced to heat exchanger 40, wherein it subsequently liquefies to produce LNG 6. In one embodiment, first portion of the LNG 146 can be removed from LNG 6, expanded in second valve V2, and then warmed in heat exchanger 40, thereby providing additional refrigeration, to produce warm natural gas stream 108. The remaining portion can then be expanded across third valve V3, thereby producing low pressure LNG 148.

Refrigeration for the system is provided by two sources. The first refrigeration source can be via a conventional nitrogen refrigeration cycle 50. Nitrogen gas is compressed in nitrogen recycle compressor 10, cooled in cooler 11, compressed further in booster of first turbine booster 20, cooled in cooler 21, then further compressed in booster of second turbine booster 25 before being cooled again in cooler 26. The resulting compressed nitrogen is then cooled in heat exchanger 40, wherein a first portion is removed and expanded in turbine of second turbine booster 25 and the remaining portion is removed and expanded in turbine of first turbine booster 20. The resulting expanded nitrogen streams are then introduced to heat exchanger 40, where they are warmed via indirect heat exchange against the natural gas and other nitrogen streams.

The second refrigeration source is provided by using the excess pressure differential of the high pressure natural gas. In this embodiment, second portion of the natural gas stream 106 is split from high pressure natural gas 2, and then purified in second purification unit 131 of at least water and potentially mercury to produce purified second portion of the natural gas 132. While the embodiment shown in FIG. 2 includes two separate purification units, it is possible to use a single purification unit to fully purify the entire natural gas stream prior to splitting the natural gas into two streams. However, it is preferable to split the streams prior to purification since the natural gas used to provide refrigeration (i.e., the portion not liquefied), does not need to have carbon dioxide removed, since the natural gas turbine outlet stream 144 is at a sufficiently warm temperature such that carbon dioxide will not freeze within this stream. In another embodiment, units 130 and 131 may be combined into a single unit, and the moisture free stream (e.g., 132) is removed at an intermediate location of the vessel and the moisture and CO₂ free stream (e.g., 104) is removed from the end of the vessel opposite the feed location.

Purified second portion of the natural gas 132 is then compressed in booster of natural gas turbine booster 120, cooled in cooler 140 to produce compressed natural gas stream 142. Compressed natural gas stream 142 can then be partially cooled in heat exchanger 40, before being expanded in turbine of natural gas turbine booster 120 to form cold natural gas stream 144. Alternatively, in an embodiment not shown, natural gas stream 142 can be sent, prior to cooling, directly to natural gas turbine 120 for expansion. This can help limit the temperature of 144 to avoid heavy hydrocarbon condensation and potential solidification. Cold natural gas stream 144 is then reintroduced to heat exchanger 40, wherein it is warmed via indirect heat exchange and collected as warm natural gas stream 108 from the warm end of the heat exchanger. In one embodiment, cold natural gas stream 144 can be combined with heavy hydrocarbons 8 and optionally first portion of the LNG 146 within the heat exchanger, or the different streams can warm individually within the heat exchanger and be combined following their warming.

The booster of natural gas turbine booster 120 can be located at many different locations depending on the natural gas source and return pressures. For example, it may be located at 1) the NG stream to be expanded (FIG. 2) if the feed pressure and/or return pressure are low, 2) the total natural gas feed flow before splitting the flow to be expanded and flow to be liquefied (FIG. 3), or 3) on the discharge of the turbine at the warm end of the exchanger (e.g., stream 108) in the case of high natural gas feed pressure and high natural return pressure (not shown), or 4) on the natural gas stream to be liquefied (e.g., stream 104) if the feed pressure is low (not shown). Alternatively the turbine may be used to drive an electrical generator or dissipated by oil brake (not shown).

A comparison of the embodiment shown in FIGS. 1 and 2 can be found in Table I below:

TABLE I Comparison of Energy Requirements for FIG. 1 and FIG. 2 Base (Typical LNG LNG production by production by N2 NG letdown and N2 cycle) FIG. 1 cycle. FIG. 2 NG supply 32 bara 342 1019 (mtd) NG to letdown 5.6 0 677 bara (mtd) LNG Production 342 342 (mtd) N2 cycle power input 7155 4158 (kW) LNG Specific power 502 292 (kWh/mt) Power Reduction (%) — 42% LIN production — — (mtd) LIN Specific Power — — (kWh/mt)

In the setup shown of FIG. 2, the power required to produce 342 mtd of LNG is reduced to approximately 4158 kW, meaning the specific power of this setup is approximately 292 kWh/mt. As such, this represents a decrease of approximately 42% in power requirements.

Regarding FIG. 3, a process flow diagram of an embodiment for the co-production of liquid nitrogen and LNG using a nitrogen refrigeration cycle in combination with natural gas letdown. In FIG. 3, natural gas can be acquired from a natural gas source, compressed in natural gas booster 101 to produce high pressure natural gas 2. High pressure natural gas 2 is preferably split into two portions, with one portion being liquefied and the other portion providing a portion of the refrigeration used to cool and liquefy the natural gas. First portion of the natural gas stream 102 is purified in first purification unit 130, wherein acid gases, water and mercury are preferably removed. Preferably, any impurity within the natural gas that would damage or solidify prior to the natural gas liquefying is removed in first purification unit 130. The resulting purified first portion of the natural gas stream 104 is then withdrawn from first purification unit 130 and introduced to heat exchanger 40 for liquefaction therein. In embodiments in which the natural gas feed contains heavy hydrocarbons, it is preferable to withdraw purified first portion of the natural gas 104 from an intermediate section of heat exchanger 40 and separate the heavy hydrocarbons 8 using gas liquid separator 5. Alternatively, the gas-liquid separator may be replaced by a distillation column or other separation devices known in the art. Instead of collecting heavy hydrocarbons 8 separately as shown in FIG. 1, heavy hydrocarbons 8 may be expanded and then warmed in heat exchanger 40. The resulting warmed stream can be combined with cold natural gas stream 144 within heat exchanger 40. This advantageously captures some of the cold energy from heavy hydrocarbons 8, and if warm natural gas stream 108 is subsequently used for fuel, it also provides additional energy for that purpose.

Vaporized natural gas from gas liquid separator 5 is reintroduced to heat exchanger 40, wherein it subsequently liquefies to produce LNG 6. While not shown specifically in FIG. 3, as in FIG. 2, in one embodiment, first portion of the LNG 146 can be removed from LNG 6, expanded in second valve V2, and then warmed in heat exchanger 40, thereby providing additional refrigeration, to produce warm natural gas stream 108. The remaining portion can then be expanded across third valve V3, thereby producing second portion of the LNG 148. In the embodiment shown in FIG. 3, all of LNG 6 is expanded in valve V3 and used as product.

Refrigeration for the system is provided by two sources. The first refrigeration source can be via a conventional nitrogen refrigeration cycle 50. Nitrogen gas is compressed in nitrogen recycle compressor 10, cooled in cooler 11, compressed further in booster of first turbine booster 20, cooled in cooler 21, then further compressed in booster of second turbine booster 25 before being cooled again in cooler 26. The resulting compressed nitrogen is then cooled in heat exchanger 40, wherein a first portion is removed and expanded in turbine of second turbine booster 25, a second portion is removed and expanded in turbine of first turbine booster 20. The resulting expanded nitrogen streams are then introduced to heat exchanger 40, where they are warmed via indirect heat exchange against the natural gas and other nitrogen streams.

The second refrigeration source is provided by using the excess pressure differential of the high pressure natural gas. In this embodiment, second portion of the natural gas stream 106 is split from high pressure natural gas 2, and then purified in second purification unit 131 of at least water and preferably mercury to produce purified second portion of the natural gas 132. While the embodiment shown in FIG. 3 includes two separate purification units, it is possible to use a single purification unit to fully purify the entire natural gas stream prior to splitting the natural gas into two streams. However, it is preferable to split the streams prior to purification since the natural gas used to provide refrigeration (i.e., the portion not liquefied), does not need to have carbon dioxide removed, since the natural gas turbine outlet stream 144 is at a sufficiently warm temperature such that carbon dioxide will not freeze within this stream. In another embodiment, units 130 and 131 may be combined into a single unit, and the moisture free stream (e.g., 132) is removed at an intermediate location of the vessel and the moisture and CO₂ free stream (e.g., 104) is removed from the end of the vessel opposite the feed location.

Purified second portion of the natural gas 132 can then be partially cooled in heat exchanger 40, before being expanded in turbine 121 of natural gas turbine booster 120 to form cold natural gas stream 144. Alternatively, in an embodiment not shown, purified second portion of the natural gas stream 132 can be sent, prior to cooling, directly to natural gas turbine 121 for expansion. This can help limit the temperature of 144 to avoid heavy hydrocarbon condensation and potential solidification. Cold natural gas stream 144 is then reintroduced to heat exchanger 40, wherein it is warmed via indirect heat exchange and collected as warm natural gas stream 108 from the warm end of the heat exchanger. In one embodiment, cold natural gas stream 144 can be combined with heavy hydrocarbons 8 within the heat exchanger, or the different streams can warm individually within the heat exchanger and be combined following their warming.

The booster 101 of natural gas turbine booster 120 can be located at many different locations depending on the natural gas source and return pressures. For example, it may be located at 1) the NG stream to be expanded (FIG. 2) if the feed pressure and/or return pressure are low, 2) the total natural gas feed flow before splitting the flow to be expanded and flow to be liquefied (FIG. 3), or 3) on the discharge of the turbine at the warm end of the exchanger (e.g., stream 108) in the case of high natural gas feed pressure and high natural return pressure (not shown), or 4) on stream to be liquefied (e.g., stream 104) if the feed pressure is low (not shown). Alternatively the turbine may be used to drive an electrical generator or dissipated by oil brake (not shown).

The primary difference between the embodiment of FIG. 2 and the embodiment of FIG. 3 is that in FIG. 3, low pressure gaseous nitrogen is introduced as feed into the nitrogen refrigeration cycle and LIN is coproduced with LNG. In one particular embodiment, gaseous nitrogen (“GAN”) is introduced into, and compressed by, nitrogen compressor 15 before being cooled in cooler 16 and then added to the refrigeration cycle. Those of ordinary skill in the art will recognize that the nitrogen compressor 15 can be optional, since its use can be dependent on the pressure of the GAN feed stream. In another embodiment, a third portion of the cooled nitrogen is removed from the heat exchanger 40, subcooled in nitrogen subcooler 45, and expanded across valve V4 before being introduced to nitrogen gas liquid separator 55. Nitrogen vapor 57 is withdrawn from the top of nitrogen gas liquid separator 55 and then warmed in heat exchanger 40, wherein it is then recompressed by nitrogen compressor 15 before again rejoining the refrigeration cycle. Liquid nitrogen is withdrawn from the bottom of nitrogen gas liquid separator 55 and preferably one portion 51 is sent to be vaporized in subcooler 45, while the other portion 52 is sent to a liquid nitrogen storage tank (not shown).

As such, FIG. 3 provides for an embodiment in combining LIN+LNG+natural gas letdown. As before, the nitrogen refrigeration cycle includes a recycle compressor, and at least one turbine booster. However, because it produces LIN (e.g., removes nitrogen molecules from the loop), it also includes a step of adding gaseous nitrogen feed to the system. In the embodiment shown in FIG. 3, the gaseous nitrogen makeup is at low pressure, and therefore it also includes a nitrogen feed compressor, as well as a subcooler to provide liquid nitrogen product. As in other embodiments, the natural gas supply is split between a flow to be liquefied and a flow to be expanded back to low pressure. As noted previously, the natural gas booster 101 may be located at various locations depending on the flow ratio and pressure of the natural gas feed and letdown pressures used.

Regarding FIG. 4, a process flow diagram of an embodiment having a partial integration of a nitrogen liquefier with a natural gas liquefier is shown. In FIG. 4, natural gas can be acquired from a natural gas source, compressed in natural gas booster 101 to produce high pressure natural gas 2. High pressure natural gas 2 is preferably split into two portions, with one portion being liquefied and the other portion providing a portion of the refrigeration used to liquefy the natural gas. First portion of the natural gas stream 102 is purified in first purification unit 130, wherein acid gases, water and mercury are preferably removed. Preferably, any impurity within the natural gas that would damage equipment or solidify prior to the natural gas liquefying is removed in first purification unit 130. The resulting purified first portion of the natural gas stream 104 is then withdrawn from first purification unit 130 and introduced to heat exchanger 440 for liquefaction therein. In embodiments in which the natural gas feed contains heavy hydrocarbons, it is preferable to withdraw purified first portion of the natural gas 104 from an intermediate section of heat exchanger 440 or before entering exchanger 440 and separate the heavy hydrocarbons 8 using gas liquid separator 5 or distillation column. In one embodiment, heavy hydrocarbons 8 may be expanded and then warmed in heat exchanger 440. The resulting warmed stream can be combined with other natural gas streams (e.g., cold natural gas stream 144) within heat exchanger 440. This advantageously captures some of the cold energy from heavy hydrocarbons 8, and if warm natural gas stream 108 is subsequently used for fuel, it also provides additional energy for that purpose. Vaporized natural gas from gas liquid separator 5 is reintroduced to heat exchanger 440, wherein it subsequently liquefies to produce LNG 6.

Refrigeration for the system can be provided by three sources, a first nitrogen refrigeration cycle 50, a second nitrogen refrigeration cycle 450, and by expansion of high pressure natural gas. In first nitrogen refrigeration cycle 50, nitrogen gas coming from first nitrogen refrigeration cycle 50 and second nitrogen refrigeration cycle 450 is compressed in shared nitrogen recycle compressor 410, and cooled in cooler 411. The resulting compressed nitrogen is then split into two streams, with a first portion going to first nitrogen refrigeration cycle 50 and the second portion going to second nitrogen refrigeration cycle 450.

With respect to first nitrogen refrigeration cycle 50, the nitrogen can be compressed further in booster of first turbine booster 20, cooled in cooler 21, further compressed in booster of second turbine booster 25 before being cooled again in cooler 26. The resulting compressed nitrogen is then cooled in heat exchanger 40, wherein a first portion is removed and expanded in turbine of second turbine booster 25, a second portion is removed and expanded in turbine of first turbine booster 20. The resulting expanded nitrogen streams are then introduced to heat exchanger 40, where they are warmed via indirect heat exchange against the natural gas and other nitrogen streams, and then sent back to shared nitrogen recycle compressor 410.

As in FIG. 3, the embodiment of FIG. 4 also includes low pressure gaseous nitrogen introduced as feed and LIN is coproduced. Gaseous nitrogen (GAN) is introduced into, and compressed by, nitrogen compressor 15 before being cooled in cooler 16 and then added to the refrigeration cycle. Those of ordinary skill in the art will recognize that the nitrogen compressor 15 can be optional, since its use can be dependent on the pressure of the GAN feed stream. Additionally, the remaining portion of the compressed nitrogen is removed from the heat exchanger 40, subcooled in nitrogen subcooler 45, and expanded across valve V4 before being introduced to nitrogen gas liquid separator 55. Nitrogen vapor 57 is withdrawn from the top of nitrogen gas liquid separator 55 and then warmed in heat exchanger 40, wherein it is then recompressed by nitrogen compressor 15 before again rejoining the refrigeration cycle. Liquid nitrogen is withdrawn from the bottom of nitrogen gas liquid separator 55 then split into first portion 51 which is vaporized in subcooler 45 to provide heat exchange for the LIN subcooling and second portion 52 as LIN production preferably sent to a storage tank (not shown).

The second refrigeration source can be second nitrogen refrigeration cycle 450, which is comprised of shared nitrogen recycle compressor 410, shared cooler 411, and non-shared equipment such as third turbine booster 420, cooler 421, fourth turbine booster 425, and cooler 426.

The third source of refrigeration is provided by using available excess pressure differential of high pressure natural gas. In this embodiment, second portion of the natural gas stream 106 is split from high pressure natural gas 2, and then purified in second purification unit 131 of at least water and preferably mercury to produce purified second portion of the natural gas 132. While the embodiment shown in FIG. 4 includes two separate purification units, it is possible to use a single purification unit to fully purify the entire natural gas stream prior to splitting the natural gas into two streams. However, it is preferable to split the streams prior to purification since the natural gas used to provide refrigeration (i.e., the portion not liquefied), does not need to have carbon dioxide removed, since the natural gas turbine outlet stream 144 is at a sufficiently warm temperature such that carbon dioxide will not freeze within this stream. Alternatively, units 130 and 131 may be combined into a single unit such that the moisture free stream 132 is removed at an intermediate location of the vessel and the moisture and CO₂ free stream 104 is removed from the end of the vessel opposite the feed 2 location.

Purified second portion of the natural gas 132 may be partially cooled in heat exchanger 440, before being expanded in natural gas turbine 121 to form cold natural gas stream 144. Alternatively, stream 132 can be sent, prior to cooling in the heat exchanger, to turbine 121 for expansion to limit the temperature of 144 due to CO₂ freezing or heavy hydrocarbon condensation. Cold natural gas stream 144 is then reintroduced to heat exchanger 440, wherein it is warmed via indirect heat exchange and collected as warm natural gas stream 108 from the warm end of the heat exchanger. In one embodiment, cold natural gas stream 144 can be combined with heavy hydrocarbons 8 within the heat exchanger, or the two streams can warm individually within the heat exchanger and be combined following their warming.

The booster 101 of natural gas turbine booster 120 can be located at many different locations depending on the natural gas source and return pressures. For example, it may be located at 1) the NG stream to be expanded (FIG. 2) if the feed pressure and/or return pressure are low, 2) the total natural gas feed flow before splitting the flow to be expanded and flow to be liquefied (FIG. 3), or 3) on the discharge of the turbine at the warm end of the exchanger (e.g., 108) in the case of high natural gas feed pressure and high natural return pressure (not shown), or 4) on stream to be liquefied (e.g., 104) if the feed pressure is low (not shown). Alternatively the turbine may be used to drive an electrical generator or dissipated by oil brake (not shown).

As noted above, the embodiment of FIG. 4 preferably includes a stand-alone nitrogen liquefier 350, that shares a common nitrogen recycle compressor (e.g., 410), with the second nitrogen refrigeration cycle 450. As such, such an embodiment can advantageously produce LIN and LNG at locations that have both a nitrogen liquefaction unit and access to natural gas.

The embodiment of FIG. 4 has a 12% efficiency improvement compared to the embodiment shown in FIG. 3, primarily due to the additional turbine boosters which can be positioned at temperatures in the cycle to independently optimize the LNG and LIN trains.

Additionally, the shared recycle compressor 410 provides a lower capital cost compared to an independent nitrogen liquefier plus independent LNG plant, since the embodiment effectively eliminates one recycle compressor, which typically is the largest capital cost equipment of the system. In addition, there is a small efficiency improvement due to a single, large machine compared to two, small machines. Similarly as indicated before, the location of the booster for the natural gas letdown can vary with natural gas source and letdown pressure.

A comparison of the embodiments shown in FIGS. 1-4 can be found in Table II below.

TABLE II Comparison Data for FIGS. 1-4 LNG + LIN Base (Typical LNG LNG production by production by NG LNG + LIN production production by N2 NG letdown and N2 letdown and N2 by NG letdown and N2 cycle) FIG. 1 cycle. FIG. 2 cycle (FIG. 3) cycle (FIG. 4) NG supply 32 bara 342 1019 1019 1019 (mtd) NG to letdown 5.6 0 677 677 677 bara (mtd) LNG Production 342 342 342 342 (mtd) N2 cycle power input 7155 4158 10555 9974 (kW) LNG Specific power 502 292 353 313 (kWh/mt) Power Reduction (%) — 42% 30% 38% LIN production — — 301 301 (mtd) LIN Specific Power — — 440 440 (kWh/mt)

In an optional embodiment, fourth gas stream 351 can be cooled and/or liquefied within heat exchanger 40 to produce cooled/liquefied fourth gas stream 352. In one embodiment, fourth gas stream 351 is selected from the group consisting of natural gas; ethane; ethylene; acetylene; C₃-C₆ alkanes, alkenes and alkynes; nitrogen; hydrogen; and helium. In embodiments in which gas stream 351 is hydrogen or helium, gas stream 352 is preferably not liquefied. Otherwise, cooled stream 352 is preferably liquefied. Advantageously, this optional embodiment allows for three separate gases to be liquefied (e.g., streams 52, 352 and 6).

The embodiments shown in FIG. 3 and FIG. 4 are preferably located near, on, or have access to an industrial site with a large constant letdown flow of natural gas (e.g., a cogen unit, or steam methane reformer facility), as well as a source of nitrogen (e.g., near an air separation unit “ASU” or nitrogen pipeline). Nitrogen is often available near an ASU as they are commonly designed for O₂ production. Nitrogen may be extracted with a small cost to the ASUs precooling system.

The embodiment shown in FIG. 4 includes a specific embodiment of producing LNG and LIN, however, the invention is not to be so limited. Instead, an embodiment of the invention can include liquefaction of a first gas and a second gas, through the use of two refrigeration cycles, in which the two refrigeration cycles share a common recycle compressor. In a preferred embodiment, the refrigeration cycles are nitrogen refrigeration cycles. In one embodiment, the two liquefiers could each produce either LIN or LNG or liquid hydrogen or liquid helium or any type of other industrial gases. In another embodiment, either or both of the liquefiers may have an expansion device configured to expand a higher pressure gas source.

Embodiments of the invention can have wide applications in the industry. For example, an embodiment of the invention may include identifying an underutilized liquefaction system, and then adding a second liquefier nearby (e.g., an LNG liquefier). The original liquefier can be slightly modified in order to allow for its previously underutilized recycle compressor to provide compression for both refrigeration cycles. This allows for the new liquefier to produce its liquid in a much more efficient manner. In another embodiment, the second liquefaction unit is preferably located nearby a high and low pressure pipeline network (e.g., natural gas pipeline) such that the system is able to use the refrigeration from expansion of the natural gas.

In another embodiment, two new liquefiers can be built to satisfy a market demand. For example, the first liquefier can be a nitrogen liquefaction unit and the second liquefier can be a natural gas liquefaction unit, both using nitrogen refrigeration cycles. It can be economically advantageous that at least one of the liquefiers is a standardized plant (e.g., a modular type design that can be designed and produced in bulk). In many cases, the capacity which the standardized plant has been designed for is greater than the capacity needed for this specific application. A similar concept could apply to the relocation of an existing liquefier. Therefore, the second liquefier can be built such that its refrigeration cycle uses the same recycle compressor as the one from the first liquefier. It is also common that such liquefaction plants are located near an industrial area, therefore benefiting from a wide natural gas pipeline network. One or both liquefiers would benefit from adding a natural gas expansion refrigeration to each nitrogen refrigeration cycle, as described herein.

Similarly, if the standardized plant were undersized for a particular application (e.g., produce liquid nitrogen), the second liquefaction unit could be designed to make up the difference. In this embodiment, the second liquefaction unit could be configured to create both a liquid nitrogen product, as well as an LNG product.

Operational problems can occur when the natural gas turbine drives an electric generator without extracting the refrigeration energy of expansion. Furthermore, in some instances, the flow rate and pressures of the natural gas can often fluctuate. This can cause issues with respect to fluctuations in produced energy, since electrical systems are not always able to accept the resulting fluctuations of electricity sent to the grid from the generator. Similarly, the resulting fluctuations in cold created by the natural gas expansion can yield fluctuations in other utilities.

In certain embodiments of the invention, the above referenced problems can be mitigated through the use of an LNG and/or LIN storage tank, as the storage tank provides a buffer for the fluctuations of the refrigeration balance. For example, minor fluctuations in natural gas conditions can be accounted for by adjusting the load of the nitrogen refrigeration cycle and the quantity of LNG and/or LIN being liquefied. Large or long term fluctuations can be accounted for by stopping the liquefier and compensating by the tank level. In addition, significant short term fluctuations can be accounted for by adjusting a bypass valve to allow high pressure natural gas to bypass the liquefier and going straight to the MP GAN stream (not shown). In another embodiment, the method can include monitoring various process conditions (e.g., pressure, flow rate, gas composition, etc. . . . ) of the natural gas source, and/or streams downstream of the natural gas source. Based on these monitored process conditions, various set points can be adjusted in order to further optimize the system. For example, a set point that can be adjusted can include expansion ratio for the various turbines, along with flow rates of various streams throughout. In one embodiment, the set points for the flow rate and inlet pressure to the natural gas turbine can be controlled within an acceptable operating range of the liquefaction equipment by adjustment of the natural gas bypass valve and/or a turbine inlet control valve. In one embodiment, the method can include a central process controller that is configured to receive the various monitored process conditions and then determine whether a selected set point should be adjusted based on the monitored process conditions. The monitoring devices can communicate with the controller via all known methods, for example, both wirelessly and via wired electrical communication.

FIG. 5 provides for a process flow diagram with liquid nitrogen production being supplemented with refrigeration from letdown of natural gas. The additional energy provided by the natural gas letdown reduces the power and size of the nitrogen refrigeration cycle for a fixed LIN production depending on the amount of energy which can be removed from the natural gas letdown (i.e., flow and pressure ratio of the NG letdown).

In this type of embodiment, it is preferable that the system be proximate to a nitrogen source (e.g., ASU with available nitrogen production, or other small dedicated nitrogen generator, or nitrogen pipeline) as well as a source of pressurized natural gas suitable for letdown. While it is understood that there will be variations in the natural gas flow and pressure, the liquefier can accommodate some of these variations by a corresponding adjustment in LIN production and or power from the nitrogen refrigeration cycle.

The method shown in FIG. 5 has one natural gas turbine booster for the warm section of the exchanger and one nitrogen turbine booster for the cold section. However, for improved efficiency and flexibility, an additional warm turbine booster (as shown in FIG. 2) can be included in certain embodiments of the invention.

With respect to purification, water should be removed and depending on natural gas composition, pressure and temperature prior to natural gas expansion, acid gases such as CO₂, and other impurities which freeze at colder temperatures may be removed from the natural gas as well. The natural gas may be cooled before being expanded and can reach a temperature of approximately −60° C. to −100° C. before entering the heat exchanger, is re-warmed and returned to the low pressure header. Since CO₂ will only freeze at lower temperatures, it is not required to remove CO₂ from the stream being expanded.

Since the liquefier is intended to be in industrial facilities with constant natural gas letdown, nitrogen source, etc, these facilities often have much less impurities in the feed natural gas. For example odorization (addition of sulfur containing mercaptans) is not used in these areas. Therefore, the purification system maybe simplified compared to a similar unit installed at a non-industrial site.

Those of ordinary skill in the art will recognize that other types of refrigeration cycles may be used. Therefore, embodiments of the invention are not intended to be limited to the particular refrigeration cycles shown and described within the detailed specification and in the accompanying figures. Additionally, while the embodiments shown in the figures and discussed herein, typically show that the natural gas expansion turbine can be connected to a natural gas booster, certain embodiments of the invention are not intended to be so limited. Rather, in certain embodiments of the invention, the natural gas expansion turbine 121 can drive a booster that is located within one of the refrigeration cycles, for example the nitrogen refrigeration cycle. In this embodiment, the booster can be configured to compress a refrigeration fluid (for example, nitrogen) within the refrigeration cycle.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.

All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited. 

We claim:
 1. A method for the production of liquefied natural gas (“LNG”), the method comprising the steps of: a) providing a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle is configured to provide refrigeration within a heat exchanger; b) purifying a first natural gas stream in a first purification unit to remove a first set of impurities to produce a purified first natural gas stream; c) cooling and liquefying the first natural gas stream in the heat exchanger using the refrigeration from the nitrogen refrigeration cycle to produce an LNG stream, wherein the first natural gas stream has an LNG refrigeration requirement, wherein the LNG stream is liquefied at a first pressure P_(H); d) purifying a second natural gas stream in a second purification unit to remove a second set of impurities to produce a purified second natural gas stream; e) partially cooling the second natural gas stream in the heat exchanger; f) withdrawing the partially cooled second natural gas stream from an intermediate section of the heat exchanger; g) expanding the partially cooled second natural gas stream to a medium pressure P_(M) in a natural gas expansion turbine to form a cold natural gas stream, wherein the medium pressure P_(M) is at a pressure lower than the first pressure P_(H); and h) warming the cold natural gas stream in the heat exchanger by heat exchange against the first natural gas stream to produce a warm natural gas stream at the warm end of the heat exchanger, wherein the natural gas expansion turbine drives a first booster, wherein the LNG refrigeration requirement is supplied by a combination of refrigeration from the nitrogen refrigeration cycle and step h).
 2. The method as claimed in claim 1, wherein the first booster is configured to compress the second natural gas stream or a stream derived from the second natural gas stream.
 3. The method as claimed in claim 1, wherein the first booster is configured to compress a stream selected from the group consisting of the first natural gas stream, the first purified natural gas stream, the second natural gas stream, the purified second natural gas stream, the partially cooled natural gas stream, the warm natural gas stream, and a nitrogen fluid within the nitrogen refrigeration cycle.
 4. The method as claimed in claim 1, wherein the first set of impurities has a freezing point at or above the liquefaction temperature of methane at the first pressure P_(H).
 5. The method as claimed in claim 1, wherein the second set of impurities comprises water.
 6. The method as claimed in claim 1, wherein the nitrogen refrigeration cycle comprises a recycle compressor, a turbine, a booster and a plurality of coolers, wherein the turbine and booster are configured such that the turbine is configured to power the booster.
 7. The method as claimed in claim 1, wherein the first natural gas stream and the second natural gas stream come from the same natural gas source.
 8. The method as claimed in claim 7, wherein the natural gas source is a natural gas pipeline having a pressure between 15 and 100 bara.
 9. The method as claimed in claim 1, wherein the first natural gas stream comes from a first natural gas source, and the second natural gas stream comes from a second natural gas source, wherein the first and second natural gas sources are different sources.
 10. The method as claimed in claim 9, wherein the first natural gas source comprises a natural gas pipeline.
 11. The method as claimed in claim 10, wherein the natural gas pipeline has a pressure between 15 and 100 bara.
 12. The method as claimed in claim 1, wherein the first purification unit and the second purification unit are the same unit.
 13. The method as claimed in claim 1, wherein the first purification unit and the second purification unit are separate units, wherein the first purification unit is configured to remove at least water and carbon dioxide, and wherein the second purification unit is configured to remove at least water.
 14. A method for the production of liquefied natural gas (“LNG”), the method comprising the steps of: a) providing a nitrogen refrigeration cycle; b) cooling and liquefying a first natural gas stream in a heat exchanger by heat exchange with nitrogen from the nitrogen refrigeration cycle to produce an LNG stream, wherein the LNG stream is liquefied at a first pressure; c) expanding a second natural gas stream to a second pressure to produce an expanded natural gas stream; and d) warming the expanded natural gas stream in the heat exchanger to produce a warmed natural gas stream, wherein step d) provides a portion of the refrigeration used to cool and liquefy the first natural gas stream.
 15. The method as claimed in claim 14, wherein the first natural gas stream comes from a first natural gas source, and the second natural gas stream comes from a second natural gas source, wherein the first and second natural gas sources are different sources.
 16. The method as claimed in claim 14, wherein the first natural gas liquefied in step b) is derived from the expanded natural gas stream, wherein the first pressure and the second pressure are about the same.
 17. A method for the production of liquefied natural gas (“LNG”), the method comprising the steps of: a) providing a high pressure natural gas stream; b) splitting the high pressure natural gas stream into a first portion and a second portion; c) cooling and liquefying the first portion of the high pressure natural gas stream to produce an LNG stream; d) providing a first portion of refrigeration via a nitrogen refrigeration cycle, wherein the nitrogen refrigeration cycle comprises a recycle compressor, a turbine, a booster and a plurality of coolers, wherein the turbine and booster are configured such that the turbine is configured to power the booster; e) providing a second portion of refrigeration by expanding the second portion of the high pressure natural gas; and f) using the first portion of refrigeration and the second portion of refrigeration to achieve the cooling and liquefaction of the first portion of the high pressure natural gas stream in step c). 